1. Field of the Invention
This invention generally relates to the testing of underground formations or reservoirs. More particularly, this invention relates to an apparatus and methods for sampling and testing a formation fluid.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, well boreholes are drilled by rotating a drill bit attached at a drill string end. The drill string may be a jointed rotatable pipe or a coiled tube. A large portion of the current drilling activity involves directional drilling, i.e., drilling boreholes deviated from vertical and/or horizontal boreholes, to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from earth formations. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional downhole instruments, known as measurement-while-drilling (MWD) or logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine formation geology and formation fluid conditions during the drilling operations.
Pressurized drilling fluid (commonly known as the “mud” or “drilling mud”) is pumped into the drill pipe to rotate the drill motor, to provide lubrication to various members of the drill string including the drill bit and to remove cuttings produced by the drill bit. The drill pipe is rotated by a prime mover, such as a motor, to facilitate directional drilling and to drill vertical boreholes. The drill bit is typically coupled to a bearing assembly having a drive shaft which in turn rotates the drill bit attached thereto. Radial and axial bearings in the bearing assembly provide support to the drill bit against these radial and axial forces.
Boreholes are usually drilled along predetermined paths and proceed through various formations. A drilling operator typically controls the surface-controlled drilling parameters to optimize the drilling operations. These parameters include weight on bit, drilling fluid flow through the drill pipe, drill string rotational speed (r.p.m. of the surface motor coupled to the drill pipe) and the density and viscosity of the drilling fluid. The downhole operating conditions continually change and the operator must react to such changes and adjust the surface-controlled parameters to continually optimize the drilling operations. For drilling a borehole in a virgin region, the operator typically relies on seismic survey plots, which provide a macro picture of the subsurface formations and a pre-planned borehole path. For drilling multiple boreholes in the same formation, the operator may also have information about the previously drilled boreholes in the same formation.
Typically, the information provided to the operator during drilling includes borehole pressure, temperature, and drilling parameters such as weight-on-bit (WOB), rotational speed of the drill bit and/or the drill string, and the drilling fluid flow rate. In some cases, the drilling operator is also provided selected information about the bottomhole assembly condition (parameters), such as torque, mud motor differential pressure, torque, bit bounce and whirl, etc.
Downhole sensor data are typically processed downhole to some extent and telemetered uphole by sending a signal through the drill string or by transmitting pressure pulses through the circulating drilling fluid, i.e. mud-pulse telemetry. Although mud-pulse telemetry is more commonly used, such a system is capable of transmitting only a few (1-4) bits of information per second. Due to such a low transmission rate, the trend in the industry has been to attempt to process greater amounts of data downhole and transmit selected computed results or “answers” uphole for use by the driller for controlling the drilling operations.
Commercial development of hydrocarbon fields requires significant amounts of capital. Before field development begins, operators desire to have as much data as possible in order to evaluate the reservoir for commercial viability. Despite the advances in data acquisition during drilling using the MWD systems, it is often necessary to conduct further testing of the hydrocarbon reservoirs in order to obtain additional data. Therefore, after the well has been drilled, the hydrocarbon zones are often tested with other test equipment.
One type of post-drilling test involves producing fluid from the reservoir, collecting samples, shutting-in the well, reducing a test volume pressure, and allowing the pressure to build-up to a static level. This sequence may be repeated several times at several different reservoirs within a given borehole or at several points in a single reservoir. This type of test is known as a “Pressure Build-up Test.” One important aspect of data collected during such a Pressure Build-up Test is the pressure build-up information gathered after drawing down the pressure in the test volume. From this data, information can be derived as to permeability and size of the reservoir. Moreover, actual samples of the reservoir fluid can be obtained and tested to gather Pressure-Volume-Temperature data relevant to the reservoir's hydrocarbon distribution.
Some systems require retrieval of the drill string from the borehole to perform pressure testing. The drill is removed, and a pressure measuring tool is run into the borehole using a wireline and packers for isolating the reservoir. Although wireline conveyed tools are capable of testing a reservoir, it is difficult to convey a wireline tool in a deviated borehole.
Numerous communication devices have been designed which provide for manipulation of the test assembly, or alternatively, provide for data transmission from the test assembly. Some of those designs include mud-pulse telemetry to or from a downhole microprocessor located within, or associated with the test assembly. Alternatively, a wire line can be lowered from the surface, into a landing receptacle located within a test assembly, thereby establishing electrical signal communication between the surface and the test assembly.
Regardless of the type of test equipment currently used, and regardless of the type of communication system used, the amount of time and money required for retrieving the drill string and running a second test rig into the hole is significant. Further, when a hole is highly deviated wireline conveyed test figures cannot be used because frictional force between the test rig and the wellbore exceed gravitational force causing the test rig to stop before reaching the desired formation.
A more recent system is disclosed in U.S. Pat. No. 5,803,186 to Berger et al. The '186 patent provides a MWD system that includes use of pressure and resistivity sensors with the MWD system, to allow for real time data transmission of those measurements. The '186 device enables obtaining static pressures, pressure build-ups, and pressure draw-downs with the work string, such as a drill string, in place. Also, computation of permeability and other reservoir parameters based on the pressure measurements can be accomplished without removing the drill string from the borehole.
A problem with the system described in the '186 patent relates to the time required for completing a test. During drilling, density of the drilling fluid is calculated to achieve maximum drilling efficiency while maintaining safety, and the density calculation is based upon the desired relationship between the weight of the drilling mud column and the predicted downhole pressures to be encountered. After a test is taken a new prediction is made, the mud density is adjusted as required and the bit advances until another test is taken. Different formations are penetrated during drilling, and the pressure can change significantly from one formation to the next and in short distances due to different formation compositions. If formation pressure is lower than expected, the pressure from the mud column may cause unnecessary damage to the formation. If the formation pressure is higher than expected, a pressure kick could result. Consequently, delay in providing measured pressure information to the operator results in drilling mud being maintained at too high or too low a density for maximum efficiency and maximum safety.
A drawback of the '186 patent, as well as other systems requiring fluid intake, is that system clogging caused by debris in the fluid can seriously impede drilling operations. When drawing fluid into the system, cuttings from the drill bit or other rocks being carried by the fluid may enter the system. The '186 patent discloses a series of conduit paths and valves through which the fluid must travel. It is possible for debris to clog the system at any valve location, at a conduit bend or at any location where conduit size changes. If the system is clogged, it may have to be retrieved from the borehole for cleaning causing enormous delay in the drilling operation. Therefore, it is desirable to have an apparatus with reduced risk of clogging to increase drilling efficiency.
Several formation testing tools extend a telescoping probe from the tool to the borehole wall, isolating a portion of the wall. The probe commonly has an elastomer seal on the surface in contact with the borehole wall for sealing the test volume from the rest of the annulus. The internal volume of the tool is initially filled with an incompressible fluid, typically borehole fluid. As the seal is pressed against the wall to seal, the internal volume is slightly decreased and a pressure spike occurs in the internal tool volume related to the compressibility of the fluid. Even a small change in volume can cause a substantial pressure rise, also known as a pressure spike. The pressure spike can cause damage to the formation. In addition the pressure spike creates an erroneous start pressure for the draw down sequence of the test. The pressure spike is exacerbated in small volume systems. Therefore, a need exists for a system that prevents such a pressure spike as the probe is sealed to the formation.